Active matrix display panel with ground tie lines

ABSTRACT

A display panel and a method of forming a display panel are described. The display panel may include a thin film transistor substrate including a pixel area and a non-pixel area. The pixel area includes an array of bank openings and an array of bottom electrodes within the array of bank openings. A ground line is located in the non-pixel area and an array of ground tie lines run between the bank openings in the pixel area and are electrically connected to the ground line in the non-pixel area.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/842,793, filed on Mar. 15, 2013, which is a continuation-in-part ofU.S. patent application Ser. No. 13/710,443, filed on Dec. 10, 2012,both of which are incorporated herein by reference.

BACKGROUND

1. Field

Embodiments of the present invention relate to display systems. Moreparticularly embodiments of the present invention relate to a groundingstructure for an active matrix display panel.

2. Background Information

Flat panel displays are gaining popularity in a wide range of electronicdevices. Common types of flat panel displays include active matrixdisplays and passive matrix displays. Each pixel in an active matrixdisplay panel is driven by active driving circuitry, while each pixel ina passive matrix display panel does not use such driving circuitry.High-resolution color display panels, such as modern computer displays,smart phones and televisions typically use an active matrix displaypanel structure for better image quality.

One kind of display panel that is finding commercial application is anactive matrix organic light emitting diode (AMOLED) display panel. FIG.1 is a top view illustration of a top emission AMOLED display panel.FIG. 2 is a cross-sectional side view illustration of FIG. 1 taken alongline X-X in the pixel area 104 and line Y-Y crossing the ground ring 116in the non-pixel area. The AMOLED display panel 100 illustrated in FIGS.1-2 generally includes a thin film transistor (TFT) substrate 102supporting a pixel area 104 and non-pixel area outside of the pixel area102. A TFT substrate 102 is also referred to as a backplane. A TFTsubstrate which has been further processed to additionally include thepixel area and non-pixel area is also often referred to as a backplane.Two primary TFT substrate technologies used in AMOLEDs includepolycrystalline silicon (poly-Si) and amorphous silicon (a-Si). Thesetechnologies offer the potential for fabricating the active matrixbackplanes at low temperatures (below 200° C.) directly onto flexibleplastic substrates for producing flexible AMOLED displays. The pixelarea 104 generally includes pixels 106 and subpixels 108 arranged in amatrix, and a set of TFTs and capacitors connected to each subpixel fordriving and switching the subpixels. The non-pixel area generallyincludes a data driver circuit 110 connected to a data line of eachsubpixel to enable data signals (Vdata) to be transmitted to thesubpixels, a scan driver circuit 112 connected to scan lines of thesubpixels to enable scan signals (Vscan) to be transmitted to thesubpixels, a power supply line 114 to transmit a power signal (Vdd) tothe TFTs, and a ground ring 116 to transmit a ground signal (Vss) to thearray of subpixels. As shown, the data driver circuit, scan drivercircuit, power supply line, and ground ring are all connected to aflexible circuit board (FCB) 113 which includes a power source forsupplying power to the power supply line 114 and a power source groundline electrically connected to the ground ring 116.

In the exemplary AMOLED backplane configuration an organic thin film 120and top electrode 118 are deposited over every subpixel 108 in the pixelarea 104. The organic thin film 120 may include multiple layers such asa hole injection layer, hole transport layer, light emitting layer,electron transport layer, and electron injection layer. The multiplelayers of the organic thin film 120 are typically formed over the entirepixel area 104, however, the light emitting layer is often depositedwith aid of a shadow mask only within the subpixel openings 127 and onthe bottom electrode layer 124 corresponding to the emission area forthe array of subpixels 108. A top electrode layer 118 is then depositedover the organic thin film within both the pixel area 104 and alsowithin the non-pixel area so that the top electrode 118 layer overlapsthe ground ring 116 in the in order to transfer the ground signal to thearray of subpixels. In this manner, each of the subpixels 108 can beindividually addressed with the corresponding underlying TFT circuitrywhile a uniform ground signal is supplied to the top of the pixel area104.

In the particular implementation illustrated, the TFT substrate 102includes a switching transistor T1 connected to a data line 111 from thedata driver circuit 110 and a driving transistor T2 connected to a powerline 115 connected to the power supply line 114. The gate of theswitching transistor T1 may also be connected to a scan line (notillustrated) from the scan driver circuit 112. A planarization layer 122is formed over the TFT substrate, and openings are formed to expose theTFT working circuitry. As illustrated, a bottom electrode layer 124 isformed on the planarization layer in electrical connection with the TFTcircuitry. Following the formation of the electrode layer a pixeldefining layer 125 is formed including an array of subpixel openings 127corresponding to the emission area for the array of subpixels 108,followed by deposition of the organic layer 120 and top electrode layer118 over the patterned pixel defining layer, and within subpixelopenings 127 of the patterned pixel defining layer 125. The topelectrode layer 118 additionally is formed in the non-pixel area and inelectrical connection with the ground ring 116.

The planarization layer 122 may function to prevent (or protect) theorganic layer 120 and the bottom electrode layer 124 from shorting dueto a step difference. Exemplary planarization layer 122 materialsinclude benzocyclobutene (BCB) and acrylic. The pixel defining layer 125can be formed of a material such as polyimide. The bottom electrode 124is commonly formed on indium tin oxide (ITO), ITO/Ag, ITO/Ag/ITO,ITO/Ag/indium zinc oxide (IZO), or ITO/Ag alloy/ITO. The top electrodelayer 118 is formed of a transparent material such as ITO for topemission.

While AMOLED display panels generally consume less power than liquidcrystal display (LCD) panels, an AMOLED display panel can still be thedominant power consumer in battery-operated devices. To extend batterylife, it is necessary to reduce the power consumption of the displaypanel.

SUMMARY OF THE INVENTION

A display panel and a method of forming a display panel are described.In an embodiment a display panel includes a TFT substrate including apixel area and a non-pixel area. For example, the non-pixel area maysurround the pixel area. The pixel area includes an array of bankopenings and an array of bottom electrodes within the array of bankopenings. The array of bottom electrodes may be formed on sidewalls ofthe corresponding array of bank openings, and may be reflective to thevisible wavelength. In an embodiment a post of solder material is formedon the bottom electrode within each bank opening in order to aid thebonding of a micro LED device to the bottom electrode. A ground line isformed in the non-pixel area, and an array of ground tie lines runbetween the bank openings in the pixel area and are electricallyconnected to the ground line in the non pixel area. In an embodiment,the ground line is a ground ring, and the array of ground tie lines areelectrically connected to the ground ring on opposite sides of the pixelarea.

In an embodiment a patterned insulator layer covers the array of bottomelectrodes, and an array of openings is formed in the patternedinsulator exposing the array of bottom electrodes. In this manner, thepatterned insulator layer may cover the edges of the array of bottomelectrodes. Another array of openings may also be formed in thepatterned insulator layer exposing he array of ground tie lines.

In an embodiment, an array of micro LED devices are on the array ofbottom electrodes within the corresponding array of bank openings. Forexample, the micro LED devices may be vertical micro LED devices, andmay have a maximum width of 1 μm-100 μm. A transparent passivation layercan be formed spanning sidewalls of the array of micro LED deviceswithout completely covering a top conductive contact of each micro LEDdevice. In an embodiment, an array of top electrode layers are formedover and in electrical contact with the array of micro LED devices andthe array of ground tie lines. For example, each top electrode layer mayelectrically connect a plurality of micro LED devices to a single groundtie line. The top electrode layers may also be formed of a transparentor semi-transparent material such as PEDOT or ITO. In anotherembodiment, the top electrode layer is formed over an in electricalcontact with the array of micro LED devices and the array of ground tielines through an array of openings exposing the array of ground tielines.

In an embodiment, a method of forming a display panel includestransferring an array of micro LED devices from a carrier substrate to abackplane that comprises a TFT substrate including a pixel area and anon-pixel area, where the pixel area includes an array of bank openingsand an array of bottom electrodes within the array of bank openings. TheTFT substrate also includes a ground line in the non-pixel area, and anarray of ground tie lines running between the bank openings in the pixelarea and electrically connected to the ground line in the non-pixelarea. In an embodiment a top electrode layer is deposited over all ofthe micro LED devices in the array of micro LED devices. In anembodiment, the top electrode layer spans over a plurality of the microLED devices. The top electrode layer may additionally be formed by inkjet printing or screen printing. For example a plurality of separateelectrode layers can be ink jet printed, with each separate topelectrode layer spanning over at least one of the micro LED devices andat least one of the ground tie lines. In jet printing may also includeforming the top electrode layer within an opening over one or moreground tie lines.

In an embodiment, transfer of the array of micro LED device is performedwith electrostatic principles using an array of electrostatic transferheads. Furthermore, bonding of the array of micro LED devices mayinclude the formation of an inter-metallic compound, and may includeliquefying an array of bonding layers formed on the array of bottomelectrodes. Bonding and liquefying may be accomplished in part by thetransfer of thermal energy from the array of electrostatic transferheads to the array of bonding layers formed on the array of bottomelectrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view illustration of a top emission AMOLED displaypanel.

FIG. 2 is a side-view illustration of the top emission AMOLED displaypanel of FIG. 1 taken along lines X-X and Y-Y.

FIG. 3A is a top view illustration of an active matrix display panel inaccordance with an embodiment of the invention.

FIG. 3B is a side-view illustration of the active matrix display panelof FIG. 3A taken along lines X-X and Y-Y in accordance with anembodiment of the invention.

FIG. 3C is a side-view illustration of the active matrix display panelof FIG. 3A taken along lines X-X and Y-Y in accordance with anembodiment of the invention in which ground tie lines and ground ringare formed within a patterned bank layer.

FIG. 3D is a side-view illustration of the active matrix display panelof FIG. 3A taken along lines X-X and Y-Y in accordance with anembodiment of the invention in which ground tie lines and ground ringare formed below a patterned bank layer.

FIGS. 4A-4H are cross-sectional side view illustrations for a method oftransferring an array of micro LED devices to a TFT substrate inaccordance with an embodiment of the invention.

FIGS. 5A-5C are top view illustrations for a sequence of transferring anarray of micro LED devices with different color emissions in accordancewith an embodiment of the invention.

FIG. 6A is a top view illustration of an active matrix display panelafter the formation of a top electrode layer in accordance with anembodiment.

FIG. 6B is a side-view illustration of the active matrix display panelof FIG. 6A taken along lines X-X and Y-Y in accordance with anembodiment of the invention.

FIG. 6C is a side-view illustration of the active matrix display panelof FIG. 6A taken along lines X-X and Y-Y illustrating a passivationlayer formed on patterned bank layer in accordance with an embodiment ofthe invention.

FIG. 7A is a top view illustration of an active matrix display panelafter the formation of a top electrode layer in accordance with anembodiment.

FIG. 7B is a side-view illustration of the active matrix display panelof FIG. 7A taken along lines X-X and Y-Y in accordance with anembodiment of the invention.

FIG. 8A is a top view illustration of an active matrix display panelafter the formation of separate top electrode layers in accordance withan embodiment.

FIG. 8B is a side-view illustration of the active matrix display panelof FIG. 8A taken along lines X-X and Y-Y in accordance with anembodiment of the invention.

FIG. 8C is a top schematic view illustration of a scribed top electrodelayer in accordance with an embodiment of the invention.

FIG. 9A is a top view illustration of a top emission AMOLED displaypanel including ground tie lines in accordance with an embodiment of theinvention.

FIG. 9B is a side-view illustration of the top emission AMOLED displaypanel of FIG. 9A taken along lines X-X and Y-Y in accordance with anembodiment of the invention.

FIG. 9C is a top view illustration of a top emission AMOLED displaypanel including ground tie lines in accordance with an embodiment of theinvention.

FIG. 9D is a side-view illustration of the top emission AMOLED displaypanel of FIG. 9C taken along lines X-X and Y-Y in accordance with anembodiment of the invention.

FIG. 10 is a schematic illustration of a display system in accordancewith an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention relate to display systems. Moreparticularly embodiments of the present invention relate to a groundingstructure for an active matrix display panel.

In one embodiment, an active matrix display panel includes anarrangement of ground tie lines running between bank openings in a pixelarea of the display panel. A top electrode layer can be deposited overall of the LEDs within the bank openings and in electrical contact withthe ground tie lines, or separate top electrode layers can be depositedover one or more LEDs within the bank openings and in electrical contactwith one or more ground tie lines. In one aspect, the arrangement ofground tie lines may more uniformly distribute the ground signal to thearray of LEDs on the display panel, thereby providing more uniform lightemission across the panel. In another aspect, the arrangement of groundtie lines enables reduction of power consumption of the display panel byreducing contact resistance in the electrical path from LED to groundline, where the distance of the electrical path through the topelectrode layer is reduced by connecting the top electrode layer to aground tie line of higher electrical conductivity than the top electrodelayer.

In yet another aspect, embodiments of the invention describe an activematrix display panel including wafer-based emissive micro LED devices. Amicro LED device combines the performance, efficiency, and reliabilityof wafer-based LED devices with the high yield, low cost, mixedmaterials of thin film electronics used to form AMOLED backplanes. Theterms “micro” device or “micro” LED structure as used herein may referto the descriptive size of certain devices or structures in accordancewith embodiments of the invention. As used herein, the terms “micro”devices or structures are meant to refer to the scale of 1 to 100 μm.However, it is to be appreciated that embodiments of the presentinvention are not necessarily so limited, and that certain aspects ofthe embodiments may be applicable to larger, and possibly smaller sizescales. In an embodiment, a display panel is similar to a typical OLEDdisplay panel, with a micro LED device having replaced the organic layerof the OLED display panel in each subpixel. Exemplary micro LED deviceswhich may be utilized with some embodiments of the invention aredescribed in U.S. patent application Ser. No. 13/372,222, U.S. patentapplication Ser. No. 13/436,260, U.S. patent application Ser. No.13/458,932, U.S. patent application Ser. No. 13/711,554, and U.S. patentapplication Ser. No. 13/749,647 all of which are incorporated herein byreference. The micro LED devices are highly efficient at light emissionand consume very little power (e.g., 250 mW for a 10 inch diagonaldisplay) compared to 5-10 watts for a 10 inch diagonal LCD or OLEDdisplay, enabling reduction of power consumption of the display panel.

In various embodiments, description is made with reference to figures.However, certain embodiments may be practiced without one or more ofthese specific details, or in combination with other known methods andconfigurations. In the following description, numerous specific detailsare set forth, such as specific configurations, dimensions andprocesses, etc., in order to provide a thorough understanding of thepresent invention. In other instances, well-known semiconductorprocesses and manufacturing techniques have not been described inparticular detail in order to not unnecessarily obscure the presentinvention. Reference throughout this specification to “one embodiment”means that a particular feature, structure, configuration, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the invention. Thus, the appearances ofthe phrase “in one embodiment” in various places throughout thisspecification are not necessarily referring to the same embodiment ofthe invention. Furthermore, the particular features, structures,configurations, or characteristics may be combined in any suitablemanner in one or more embodiments.

The terms “spanning”, “over”, “to”, “between” and “on” as used hereinmay refer to a relative position of one layer with respect to otherlayers. One layer “spanning”, “over” or “on” another layer or bonded“to” or in “contact” with another layer may be directly in contact withthe other layer or may have one or more intervening layers. One layer“between” layers may be directly in contact with the layers or may haveone or more intervening layers.

Referring now to FIGS. 3A-3B an embodiment is illustrated in which abackplane similar to an AMOLED backplane is modified to receive emissivemicro LED devices rather than an organic emission layer. FIG. 3A is atop view illustration of an active matrix display panel in accordancewith an embodiment, and FIG. 3B is a side-view illustration of theactive matrix display panel of FIG. 3A taken along lines X-X and Y-Y inaccordance with an embodiment of the invention. In such an embodiment,the underlying TFT substrate 102 can be similar to those in a typicalAMOLED backplane described with regard to FIGS. 1-2 including workingcircuitry (e.g. T1, T2) and planarization layer 122. Openings 131 may beformed in the planarization layer 122 to contact the working circuitry.The working circuitry can include traditional 2T1C (two transistors, onecapacitor) circuits including a switching transistor, a drivingtransistor, and a storage capacitor. It is to be appreciated that the2T1C circuitry is meant to be exemplary, and that other types ofcircuitry or modifications of the traditional 2T1C circuitry arecontemplated in accordance with embodiments of the invention. Forexample, more complicated circuits can be used to compensate for processvariation of the driver transistor and the light emitting device, or fortheir instabilities. Furthermore, while embodiments of the invention aredescribed and illustrated with regard to top gate transistor structuresin the TFT substrate 102, embodiments of the invention also contemplatethe use of bottom gate transistor structures. Likewise, whileembodiments of the invention are described and illustrated with regardto a top emission structure, embodiments of the invention alsocontemplate the use of bottom, or both top and bottom emissionstructures. In addition, embodiments of the invention are described andillustrated below specifically with regard to a high side driveconfiguration including ground tie lines and ground ring. In a high sidedrive configuration a LED may be on the drain side of a PMOS drivertransistor or a source side of an NMOS driver transistor so that thecircuit is pushing current through the p-terminal of the LED.Embodiments of the invention are not so limited may also be practicedwith a low side drive configuration in which case the ground tie linesand ground ring become the power line in the panel and current is pulledthrough the n-terminal of the LED.

A patterned bank layer 126 including bank openings 148 is then formedover the planarization layer 122. Bank layer 126 may be formed by avariety of techniques such as ink jet printing, screen printing,lamination, spin coating, CVD, and PVD. Bank layer 126 may be may beopaque, transparent, or semi-transparent to the visible wavelength. Banklayer 126 may be formed of a variety of insulating materials such as,but not limited to, photo-definable acrylic, photoresist, silicon oxide(SiO₂), silicon nitride (SiN_(x)), poly(methyl methacrylate) (PMMA),benzocyclobutene (BCB), polyimide, acrylate, epoxy, and polyester. In anembodiment, bank player is formed of an opaque material such as a blackmatrix material. Exemplary insulating black matrix materials includeorganic resins, glass pastes, and resins or pastes including a blackpigment, metallic particles such as nickel, aluminum, molybdenum, andalloys thereof, metal oxide particles (e.g. chromium oxide), or metalnitride particles (e.g. chromium nitride).

In accordance with embodiments of the invention, the thickness of thebank layer 126 and width of the bank openings 128 described with regardto the following figures may depend upon the height of the micro LEDdevice to be mounted within the opening, height of the transfer headstransferring the micro LED devices, and resolution. In an embodiment,the resolution, pixel density, and subpixel density of the display panelmay account for the width of the bank openings 128. For an exemplary 55inch television with a 40 PPI (pixels per inch) and 211 μm subpixelpitch, the width of the bank openings 128 may be anywhere from a fewmicrons to 206 μm to account for a surrounding bank structure. For anexemplary display panel with 440 PPI and a 19 μm subpixel pitch, thewidth of the bank openings 128 may be anywhere from a few microns to 14μm to account for an exemplary 5 μm wide surrounding bank structure.Width of the bank structure (i.e. between bank openings 128) may be anysuitable size, so long as the structure supports the required processesand is scalable to the required PPI.

In accordance with embodiments of the invention, the thickness of thebank layer 126 is not too thick in order for the bank structure tofunction. Thickness may be determined by the micro LED device height anda predetermined viewing angle. For example, where sidewalls of the bankopenings 128 make an angle with the planarization layer 122, shallowerangles may correlate to a wider viewing angle of the system. In anembodiment, exemplary thicknesses of the bank layer 126 may be between 1μm-50 μm.

A patterned conductive layer is then formed over the patterned banklayer 126. Referring to FIG. 3B, in one embodiment the patternedconductive layer includes bottom electrodes 142 formed within the bankopenings 148 and in electrical contact with the working circuitry. Thepatterned conductive layer may also optionally include the ground tielines 144 and/or the ground ring 116. As used herein the term ground“ring” does not require a circular pattern, or a pattern that completelysurrounds an object. Rather, the term ground “ring” means a pattern thatat least partially surrounds the pixel area on three sides. In addition,while the following embodiments are described and illustrated withregard to a ground ring 116, it is to be appreciated that embodiments ofthe invention can also be practiced with a ground line running along oneside (e.g. left, right, bottom, top), or two sides (a combination of twoof the left, right, bottom, top) of the pixel area. Accordingly, it isto be appreciated that in the following description the reference to andillustration of a ground ring, could potentially be replaced with aground line where system requirements permit.

The patterned conductive layer may be formed of a number of conductiveand reflective materials, and may include more than one layer. In anembodiment, a patterned conductive layer comprises a metallic film suchas aluminum, molybdenum, titanium, titanium-tungsten, silver, or gold,or alloys thereof. The patterned conductive layer may include aconductive material such as amorphous silicon, transparent conductiveoxides (TCO) such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO),carbon nanotube film, or a transparent conducting polymer such aspoly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline, polyacetylene,polypyrrole, and polythiophene. In an embodiment, the patternedconductive layer includes a stack of a conductive material and areflective conductive material. In an embodiment, the patternedconductive layer includes a 3-layer stack including top and bottomlayers and a reflective middle layer wherein one or both of the top andbottom layers are transparent. In an embodiment, the patternedconductive layer includes a conductive oxide-reflective metal-conductiveoxide 3-layer stack. The conductive oxide layers may be transparent. Forexample, the patterned conductive layer may include an ITO-silver-ITOlayer stack. In such a configuration, the top and bottom ITO layers mayprevent diffusion and/or oxidation of the reflective metal (silver)layer. In an embodiment, the patterned conductive layer includes aTi—Al—Ti stack, or a Mo—Al—Mo-ITO stack. In an embodiment, the patternedconductive layer includes a ITO-Ti—Al—Ti-ITO stack. In an embodiment,the patterned conductive layer is 1 μm or less in thickness. Thepatterned conductive layer may be deposited using a suitable techniquesuch as, but not limited to, PVD.

Following the formation of bottom electrodes 142, ground tie lines 144,and ground ring 116, an insulator layer 146 may then optionally beformed over the TFT substrate 102 covering the sidewalls of the patteredconductive layer. The insulator layer 146 may at least partially coverthe bank layer 126 and the reflective layer forming the bottomelectrodes 142, ground tie lines 144, and/or ground ring 116.

In an embodiment, the insulator layer 146 is formed by blanketdeposition using a suitable technique such as lamination, spin coating,CVD, and PVD, and then patterned using a suitable technique such aslithography to form openings exposing the bottom electrodes 142,openings 149 exposing the ground tie lines 149, and openings 130exposing the ground ring 116. In an embodiment, ink jet printing orscreen printing may be used to form the insulator layer 146 and openings149 without requiring lithography. Insulator layer 146 may be formed ofa variety of materials such as, but not limited to, SiO₂, SiN_(x), PMMA,BCB, polyimide, acrylate, epoxy, and polyester. For example, theinsulator layer 146 may be 0.5 μm thick. The insulator layer 146 may betransparent or semi-transparent where formed over the reflective layeron sidewalls of bottom electrode 142 within the bank openings 128 as tonot significantly degrade light emission extraction of the completedsystem. Thickness of the insulator layer 146 may also be controlled toincrease light extraction efficiency, and also to not interfere with thearray of transfer heads during transfer of the array of light emittingdevices to the reflective bank structure. As will become more apparentin the following description, the patterned insulator layer 146 isoptional, and represents one manner for electrically separatingconductive layers.

In the embodiment illustrated in FIG. 3B, the bottom electrodes 142,ground tie lines 144, and ground ring 116 can be formed of the sameconductive layer. In another embodiment, the ground tie lines 144 and/orground ring 116 can be formed of a conductive material different fromthe bottom electrodes 142. For example, ground tie lines 14 and groundring 116 may be formed with a material having a higher conductivity thanthe bottom electrodes 142. In another embodiment, ground tie lines 14and/or ground ring 116 can also be formed within different layers fromthe bottom electrodes. FIGS. 3C-3D illustrate embodiments where theground tie lines 144 and ground ring 116 can be formed within or belowthe patterned bank layer 126. For example, in the embodiment illustratedin FIG. 3C, openings 149, 130 may be formed through the patterned banklayer 126 when forming the ground tie lines 144 and ground ring 116. Inthe embodiment illustrated in FIG. 3D openings 149, 130 may be formedthrough the patterned bank layer 126 and planarization layer 122 tocontact the ground tie lines 144 and ground ring 116 which may have beenformed during formation of the working circuitry of the TFT substrate102. In such an embodiment the conductive layer used to form the bottomelectrode 142 may also optionally include via opening layers 145 and 117to further enable electrical contact of the top electrode layer yet tobe formed with the ground tie lines 144 and ground ring 116 throughopenings 149 and 130, respectively. Accordingly, it is to be appreciatedthat the embodiments illustrated in FIGS. 3A-3D are not limiting andthat a number of possibilities exist for forming the ground tie lines144 and ground ring 116, as well as openings 149, 130 to expose theground tie lines 144 and/or ground ring 116, respectively.

Still referring to embodiments illustrated in FIG. 3B-3D, a bondinglayer 140 may be formed on the bottom electrode layer 142 to facilitatebonding of a micro LED device. In an embodiment, the bonding layer 140is selected for its ability to be inter-diffused with a bonding layer onthe micro LED device (yet to be placed) through bonding mechanisms suchas eutectic alloy bonding, transient liquid phase bonding, or solidstate diffusion bonding as described in U.S. patent application Ser. No.13/749,647. In an embodiment, the bonding layer 140 has a meltingtemperature of 250° C. or lower. For example, the bonding layer 140 mayinclude a solder material such as tin (232° C.) or indium (156.7° C.),or alloys thereof. Bonding layer 140 may also be in the shape of a post,having a height greater than width. In accordance with some embodimentsof the invention, taller bonding layers 140 may provide an additionaldegree of freedom for system component leveling, such as planarity ofthe array of micro LED devices with the TFT substrate during the microLED device transfer operation and for variations in height of the microLED devices, due to the change in height of the liquefied bonding layersas they spread out over the surface during bonding, such as duringeutectic alloy bonding and transient liquid phase bonding. The width ofthe bonding layers 140 may be less than a width of a bottom surface ofthe micro LEDs to prevent wicking of the bonding layers 140 around thesidewalls of the micro LEDs and shorting the quantum well structures.

In the embodiments illustrated an arrangement of ground tie lines 144run between bank openings 128 in the pixel area 104 of the display panel100. In addition, a plurality of openings 149 expose the plurality ofground tie lines 144. The number of openings 149 may or may not have a1:1 correlation to the number of columns (top to bottom) of bankopenings 128. For example, in the embodiment illustrated in FIG. 3A, aground tie opening 149 is formed for each column of bank openings 128,however, this is not required and the number of ground tie openings 149may be more or less than the number of columns of bank openings 128.Likewise, the number of ground tie lines 144 may or may not have a 1:1correlation to the number of rows (left to right) of bank openings. Forexample, in the embodiment illustrated a ground tie line 144 is formedfor every two rows of bank openings 128, however, this is not requiredand the number of ground tie lines 144 may have a 1:1 correlation, orany 1:n correlation to the number (n) of rows of bank openings 128.

While the above embodiments have been described an illustrated withground tie lines 144 running left and right horizontally across thedisplay panel 100, embodiments are not so limited. In other embodiments,the ground tie lines can run vertically, or both horizontally andvertically to form a grid. A number of possible variations areenvisioned in accordance with embodiments of the invention. It has beenobserved that operation of AMOLED configurations such as thosepreviously illustrated and described with regard to FIGS. 1-2 may resultin dimmer emission from the subpixels in the center of the pixel area,where the subpixels are furthest from the ground ring 116, compared tothe emission from subpixels at the edges of the pixel area closer to theground ring 116. In accordance with embodiments of the invention, groundtie lines are formed between the bank openings 128 in the pixel area andare electrically connected to the ground ring 116 or ground line in thenon-display area. In this manner, the ground signal may be moreuniformly applied to the matrix of subpixels, resulting in more uniformbrightness across the display panel 100. In addition, by forming theground tie lines 144 from a material having better electricalconductivity than the top electrode layer (which is yet to be formed),this may reduce the contact resistance in the electrical ground path.

FIGS. 4A-4H are cross-sectional side view illustrations for a method oftransferring an array of micro LED devices to the TFT substrate 102 inaccordance with an embodiment of the invention. Referring to FIG. 4A, anarray of transfer heads 302 supported by a transfer head substrate 300are positioned over an array of micro LED devices 400 supported on acarrier substrate 200. A heater 306 and heat distribution plate 304 mayoptionally be attached to the transfer head substrate 300. A heater 204and heat distribution plate 202 may optionally be attached to thecarrier substrate 200. The array of micro LED devices 400 are contactedwith the array of transfer heads 302, as illustrated in FIG. 4B, andpicked up from the carrier substrate 200 as illustrated in FIG. 4C. Inan embodiment, the array of micro LED devices 400 are picked up with anarray of transfer heads 302 operating in accordance with electrostaticprinciples, that is, they are electrostatic transfer heads.

FIG. 4D is a cross-sectional side view illustration of a transfer head302 holding a micro LED device 400 over a TFT substrate 102 inaccordance with an embodiment of the invention. In the embodimentillustrated, the transfer head 302 is supported by a transfer headsubstrate 300. As described above, a heater 306 and heat distributionplate 304 may optionally be attached to the transfer head substrate toapply heat to the transfer head 302. A heater 152 and heat distributionplate 150 may also, or alternatively, optionally be used to transferheat to the bonding layer 140 on the TFT substrate 102 and/or optionalbonding layer 410 on a micro LED device 400 described below.

Still referring to FIG. 4D, a close-up view of an exemplary micro LEDdevice 400 is illustrated in accordance with an embodiment. It is to beappreciated, that the specific micro LED device 400 illustrated isexemplary and that embodiments of the invention are not limited. In theparticular embodiment illustrated, the micro LED device 400 includes amicro p-n diode 450 and a bottom conductive contact 420. A bonding layer410 may optionally be formed below the bottom conductive contact 420,with the bottom conductive contact 420 between the micro p-n diode 450and the bonding layer 410. In an embodiment, the micro LED device 400further includes a top conductive contact 452. In an embodiment, themicro p-n diode 450 includes a top n-doped layer 414, one or morequantum well layers 416, and a lower p-doped layer 418. In otherembodiments, the arrangement of n-doped and p-doped layers can bereversed. The micro p-n diodes can be fabricated with straight sidewallsor tapered sidewalls. In certain embodiments, the micro p-n diodes 450possess outwardly tapered sidewalls 453 (from top to bottom). In certainembodiments, the micro p-n diodes 450 possess inwardly tapered sidewall(from top to bottom). The top and bottom conductive contacts 420, 452.For example, the bottom conductive contact 420 may include an electrodelayer and a barrier layer between the electrode layer and the optionalbonding layer 410. The top and bottom conductive contacts 420, 452 maybe transparent to the visible wavelength range (e.g. 380 nm-750 nm) oropaque. The top and bottom conductive contacts 420, 452 may optionallyinclude a reflective layer, such as a silver layer. The micro p-n diodeand conductive contacts may each have a top surface, a bottom surfaceand sidewalls. In an embodiment, the bottom surface 451 of the micro p-ndiode 450 is wider than the top surface of the micro p-n diode, and thesidewalls 453 are tapered outwardly from top to bottom. The top surfaceof the micro p-n diode 450 may be wider than the bottom surface of thep-n diode, or approximately the same width. In an embodiment, the bottomsurface 451 of the micro p-n diode 450 is wider than the top surface ofthe bottom conductive contact 420. The bottom surface of the micro p-ndiode may also be approximately the same width as the top surface of thebottom conductive contact 420. In an embodiment, the micro p-n diode 450is several microns thick, such as 3 μm or 5 μm, the conductive contacts420, 452 are 0.1 μm-2 μm thick, and the optional bonding layer 410 is0.1 μm-1 μm thick. In an embodiment, a maximum width of each micro LEDdevice 400 is 1-100 μm, for example, 30 μm, 10 μm, or 5 μm. In anembodiment, the maximum width of each micro LED device 400 must complywith the available space in the bank opening 128 for a particularresolution and PPI of the display panel.

FIG. 4E is a cross-sectional side view illustration of an array oftransfer heads holding an array micro LED devices 400 over a TFTsubstrate 102 accordance with an embodiment of the invention. FIG. 4E issubstantially similar to the structure illustrated in FIG. 4D with theprimary difference being the illustration of the transfer of an array ofmicro LED devices as opposed to a single micro LED device within thearray of micro LED devices.

Referring now to FIG. 4F the TFT substrate 102 is contacted with thearray of micro LED devices 400. In the embodiment illustrated,contacting the TFT substrate 102 with the array of micro LED devices 400includes contacting bonding layer 140 with a micro LED device bondinglayer 410 for each respective micro LED device. In an embodiment, eachmicro LED device bonding layer 410 is wider than a corresponding bondinglayer 140. In an embodiment energy is transferred from the electrostatictransfer head assembly and through the array of micro LED devices 400 tobond the array of micro LED devices 400 to the TFT substrate 102. Forexample, thermal energy may be transferred to facilitate several typesof bonding mechanisms such as eutectic alloy bonding, transient liquidphase bonding, and solid state diffusion bonding. The transfer ofthermal energy may also be accompanied by the application of pressurefrom the electrostatic transfer head assembly.

Referring to FIG. 4G, in an embodiment, the transfer of energy liquefiesbonding layer 140. The liquefied bonding layer 140 may act as a cushionand partially compensate for system uneven leveling (e.g. nonplanarsurfaces) between the array of micro devices 400 and the TFT substrateduring bonding, and for variations in height of the micro LED devices.In the particular implementation of transient liquid phase bonding theliquefied bonding layer 140 inter-diffuses with the micro LED devicebonding layer 410 to form an inter-metallic compound layer with anambient melting temperature higher than the ambient melting temperatureof the bonding layer 140. Accordingly, transient liquid phase bondingmay be accomplished at or above the lowest liquidus temperature of thebonding layers. In some embodiments of the invention, the micro LEDdevice bonding layer 410 is formed of a material having a meltingtemperature above 250° C. such as bismuth (271.4° C.), or a meltingtemperature above 350° C. such as gold (1064° C.), copper (1084° C.),silver (962° C.), aluminum (660° C.), zinc (419.5° C.), or nickel (1453°C.), and the TFT substrate bonding layer 140 has a melting temperaturebelow 250° C. such as tin (232° C.) or indium (156.7° C.).

In this manner, the substrate 150 supporting the TFT substrate 102 canbe heated to a temperature below the melting temperature of the bondinglayer 140, and the substrate 304 supporting the array of transfer headsis heated to a temperature below the melting temperature of bondinglayer 410, but above the melting temperature of bonding layer 140. Insuch an embodiment, the transfer of heat from the electrostatic transferhead assembly through the array of micro LED devices 400 is sufficientto form the transient liquid state of bonding layer 140 with subsequentisothermal solidification as an inter-metallic compound. While in theliquid phase, the lower melting temperature material both spreads outover the surface and diffused into a solid solution of the highermelting temperature material or dissolves the higher melting temperaturematerial and solidifies as an inter-metallic compound. In a specificembodiment, the substrate 304 supporting the array of transfer heads isheld at 180° C., bonding layer 410 is formed of gold, and bonding layer140 is formed of indium.

Following the transfer of energy to bond the array of micro LED devices400 to the TFT substrate, the array of micro LED devices 400 arereleased onto the receiving substrate and the array of electrostatictransfer heads are moved away as illustrated in FIG. 4H. Releasing thearray of micro LED devices 400 may be accomplished with a variety ofmethods including turning off the electrostatic voltage sources,lowering the voltage across the electrostatic transfer head electrodes,changing a waveform of an AC voltage, and grounding the voltage sources.

Referring now to FIGS. 5A-5C, a sequence of transferring an array ofmicro LED devices 400 with different color emissions is illustrated inaccordance with an embodiment of the invention. In the particularconfiguration illustrated in FIG. 5A, a first transfer procedure hasbeen completed for transferring an array of red-emitting micro LEDdevices 400R from a first carrier substrate to the TFT substrate 102.For example, where the micro LED devices 400R are designed to emit a redlight (e.g. 620-750 nm wavelength) the micro p-n diode 450 may include amaterial such as aluminum gallium arsenide (AlGaAs), gallium arsenidephosphide (GaAsP), aluminum gallium indium phosphide (AlGaInP), andgallium phosphide (GaP). Referring to FIG. 5B, a second transferprocedure has been completed for transferring an array of green-emittingmicro LED devices 400G from a second carrier substrate to the TFTsubstrate 102. For example, where the micro LED devices 400G aredesigned to emit a green light (e.g. 495-570 nm wavelength) the microp-n diode 450 may include a material such as indium gallium nitride(InGaN), gallium nitride (GaN), gallium phosphide (GaP), aluminumgallium indium phosphide (AlGaInP), and aluminum gallium phosphide(AlGaP). Referring to FIG. 5C, a third transfer procedure has beencompleted for transferring an array of blue-emitting micro LED devices400B from a third carrier substrate to the TFT substrate 102. Forexample, where the micro LED devices 400B are designed to emit a bluelight (e.g. 450-495 nm wavelength) the micro p-n diode 450 may include amaterial such as gallium nitride (GaN), indium gallium nitride (InGaN),and zinc selenide (ZnSe).

In accordance with embodiments of the invention, the transfer heads areseparated by a pitch (x, y, and/or diagonal) that matches a pitch of thebank openings on the backplane corresponding to the pixel or subpixelarray. Table 1 provides a list of exemplary implementations inaccordance with embodiments of the invention for various red-green-blue(RGB) displays with 1920×1080 p and 2560×1600 resolutions. It is to beappreciated that embodiments of the invention are not limited to RGBcolor schemes or the 1920×1080 p or 2560×1600 resolutions, and that thespecific resolution and RGB color scheme is for illustrational purposesonly.

TABLE 1 Pixel Sub-Pixel Pixels Display Pitch pitch per inch Substrate(x, y) (x, y) (PPI) Possible transfer head array pitch 55″ (634 μm, (211μm, 40 X: Multiples or fractions of 211 μm 1920 × 1080 634 μm) 634 μm)Y: Multiples or fractions of 634 μm 10″ (85 μm, (28 μm, 299 X: Multiplesor fractions of 28 μm 2560 × 1600 85 μm) 85 μm) Y: Multiples orfractions of 85 μm  4″ (78 μm, (26 μm, 326 X: Multiples or fractions of26 μm  640 × 1136 78 μm) 78 μm) Y: Multiples or fractions of 78 μm  5″(58 μm, (19 μm, 440 X: Multiples or fractions of 19 μm 1920 × 1080 58μm) 58 μm) Y: Multiples or fractions of 58 μm

In the above exemplary embodiments, the 40 PPI pixel density maycorrespond to a 55 inch 1920×1080 p resolution television, and the 326and 440 PPI pixel density may correspond to a handheld device withRETINA® display. In accordance with embodiments of the invention,thousands, millions, or even hundreds of millions of transfer heads canbe included in a micro pick up array of a mass transfer tool dependingupon the size of the micro pick up array. In accordance with embodimentsof the invention, a 1 cm×1.12 cm array of transfer heads can include 837transfer heads with a 211 μm, 634 μm pitch, and 102,000 transfer headswith a 19 μm, 58 μm pitch.

The number of micro LED devices picked up with the array of transferheads may or may not match the pitch of transfer heads. For example, anarray of transfer heads separated by a pitch of 19 μm picks up an arrayof micro LED devices with a pitch of 19 μm. In another example, an arrayof transfer heads separated by a pitch of 19 μm picks up an array ofmicro LED devices with a pitch of approximately 6.33 μm. In this mannerthe transfer heads pick up every third micro LED device for transfer tothe backplane. In accordance with some embodiments, the top surface ofthe array of light emitting micro devices is higher than the top surfaceof the insulating layer so as to prevent the transfer heads from beingdamaged by or damaging the insulating layer (or any intervening layer)on the blackplane during placement of the micro LED devices within bankopenings.

FIG. 6A is a top view illustration of an active matrix display panel inaccordance with an embodiment after the formation of a top electrodelayer, and FIG. 6B is a side-view illustration of the active matrixdisplay panel of FIG. 6A taken along lines X-X and Y-Y in accordancewith an embodiment of the invention. In the embodiment illustrated inFIGS. 6A-6B, a top electrode layer 118 is formed over the pixel area 104including the array of micro LED devices 400 as well as in the non-pixelarea and overlapping the ground ring 116. Additionally, the topelectrode layer 118 is formed within the openings 149 and in electricalcontacts with the ground tie lines 144 running between the bank openings128 in the pixel area 104.

Referring to FIG. 6B, prior to forming the top electrode layer 118 themicro LED devices 400 are passivated within the bank openings 128 inorder to prevent electrical shorting between the top and bottomelectrode layers 118, 142, or shorting at the one or more quantum wells416. As illustrated, after the transfer of the array micro LED devices400, a passivation layer 148 may be formed around the sidewalls of themicro LED devices 400 within the array of bank openings 128. In anembodiment, where the micro LED devices 400 are vertical LED devices,the passivation layer 148 covers and spans the quantum well structure416. The passivation layer 148 may also cover any portions of the bottomelectrode layer 142 not already covered by the optional insulator layer146 in order to prevent possible shorting. Accordingly, the passivationlayer 148 may be used to passivate the quantum well structure 416, aswell as the bottom electrode layer. In accordance with embodiments ofthe invention, the passivation layer 148 is not formed on the topsurface of the micro LED devices 400, such as top conductive contact452. In one embodiment, a plasma etching process, e.g. O₂ or CF₄ plasmaetch, can be used after forming the passivation layer 148 to etch backthe passivation layer 148, ensuring the top surface of the micro LEDdevices 400, such as top conductive contacts 452, are exposed to enablethe top conductive electrode 118 layers 118 to make electrical contactwith the micro LED devices 400.

In accordance with embodiments of the invention, the passivation layer148 may be transparent or semi-transparent to the visible wavelength soas to not significantly degrade light extraction efficiency of thecompleted system. Passivation layer may be formed of a variety ofmaterials such as, but not limited to epoxy, acrylic (polyacrylate) suchas poly(methyl methacrylate) (PMMA), benzocyclobutene (BCB), polyimide,and polyester. In an embodiment, passivation layer 148 is formed by inkjet printing or screen printing around the micro LED devices 400.

In the particular embodiment illustrated in FIG. 6B, the passivationlayer 148 is only formed within the bank openings 128. However, this isnot required, and the passivation layer 148 may be formed on top of thebank structure layer 126. Furthermore, the formation of insulator layer146 is not required, and passivation layer 148 can also be used toelectrically insulate the conductive layers. As shown in the embodimentillustrated in FIG. 6C, the passivation layer 148 may also be used topassivate sidewalls of the conductive layer forming the bottom electrode142 and/or ground tie lines 144. In an embodiment, passivation layer 148may optionally be used to passivate ground ring 116. In accordance withsome embodiments, the formation of openings 149, 130 can be formedduring the process of ink jet printing or screen printing thepassivation layer 148 over the ground tie lines 144 or ground ring 116.In this manner, a separate patterning operation may not be required toform the openings.

In accordance with some embodiments of the invention a canal 151, orwell structure, can be formed within the bank layer 126 as illustratedin FIG. 6C in order to capture or prevent the passivation layer 148 fromspreading excessively and overflowing over the ground tie lines 149,particularly when the passivation layer 148 is formed using a solventsystem such as with ink jet printing or screen printing. Accordingly, insome embodiments, a canal 151 is formed within the bank layer 126between the bank opening 128 and an adjacent ground tie line 144.

Referring now to FIGS. 6B-6C, after formation of passivation layer 148 atop conductive electrode layer 118 is formed over each micro LED device400 and in electrical contact with the top contact layer 452, ifpresent. Depending upon the particular application in the followingdescription, top electrode layer 118 may be opaque, reflective,transparent, or semi-transparent to the visible wavelength. For example,in top emission systems the top electrode layer 118 may be transparent,and for bottom emission systems the top electrode layer may bereflective. Exemplary transparent conductive materials include amorphoussilicon, transparent conductive oxides (TCO) such as indium-tin-oxide(ITO) and indium-zinc-oxide (IZO), carbon nanotube film, or atransparent conductive polymer such as poly(3,4-ethylenedioxythiophene)(PEDOT), polyaniline, polyacetylene, polypyrrole, and polythiophene. Inan embodiment, the top electrode layer 118 includes nanoparticles suchas silver, gold, aluminum, molybdenum, titanium, tungsten, ITO, and IZO.In a particular embodiment, the top electrode layer 118 is formed by inkjet printing or screen printing. Other methods of formation may includechemical vapor deposition (CVD), physical vapor deposition (PVD), spincoating. The top electrode layer 118 may also be reflective to thevisible wavelength. In an embodiment, a top conductive electrode layer118 comprises a reflective metallic film such as aluminum, molybdenum,titanium, titanium-tungsten, silver, or gold, or alloys thereof, forexample for use in a bottom emission system.

In accordance with some embodiments of the invention the ground tielines 144 are more electrically conductive than the top electrode layer118. In the embodiment illustrated in FIG. 3D, the ground tie lines 144can be formed from the same metal layer used to formed the source/drainconnections or gate electrode layer to one of the transistors (e.g. T2)in the TFT substrate 102. For example, the ground tie lines 144 can beformed from a common interconnect material such as copper or aluminum,including their alloys. In the embodiments illustrated in FIGS. 3B-3C,the ground tie lines 144 may also be formed from the same material asthe bottom electrode layers 142. For example, the ground tie lines 144and bottom electrode layers 142 include a reflective material, which mayalso improve the electrical conductivity of the layers. In a specificexample, the ground tie lines 144 and bottom electrodes may include ametallic film or metal particles. In accordance with some embodiments,the top electrode layer 118 is formed of a transparent orsemi-transparent material such as amorphous silicon, transparentconductive oxides (TCO) such as indium-tin-oxide (ITO) andindium-zinc-oxide (IZO), carbon nanotube film, or a transparentconductive polymer such as poly(3,4-ethylenedioxythiophene) (PEDOT),polyaniline, polyacetylene, polypyrrole, and polythiophene, all of whichmay have a lower electrical conductivity than a conductive andreflective bottom electrode layer including a metallic film within afilm stack.

Referring back to FIG. 6A again, in the particular embodimentillustrated the top electrode layer 118 is formed over the pixel area104 including the array of micro LED devices 400 as well as in thenon-pixel area and overlapping the ground ring 116. Additionally, thetop electrode layer 118 is formed within the openings 149 and inelectrical contacts with the ground tie lines 144 running between thebank openings 128 in the pixel area 104.

Referring now to FIGS. 7A-7B an alternative embodiment is illustrated inwhich the top electrode layer 118 need only be formed over the pixelarea 104 in order to make electrical contact with the ground ring 116.In the particular embodiment illustrated in FIGS. 7A-7B, the topelectrode layer 118 is formed over the pixel area 104 and in electricalconnection with the ground tie lines 144. As illustrated, the topelectrode layer 118 may be formed within openings 149 to the ground tielines 144. In such an embodiment, since the ground tie lines 144 are inelectrical connection with the ground ring 116, it is not necessary toform the top electrode layer 118 outside of the pixel area 104. Asillustrated, the ground ring 116 may be buried beneath an electricallyinsulating layer such as such as insulator layer 146 in accordance withthe embodiments illustrated in FIGS. 7A-7B.

In accordance with embodiments of the invention the line widths of thetop electrode layers 118 can vary depending on application. For example,the line width may approach that of the pixel area 102. Alternatively,the line width may be minimal. For example, line widths as low asapproximately 15 μm may be accomplished with commercially available injet printers, and line widths as low as approximately 30 μm may beaccomplished with commercially available screen printers. Ink jetprinting may be particularly suitable in certain instances since it is anon-contact printing method. Accordingly, the line width of the topelectrode layer may be more or less than the maximum width of the microLED devices in accordance with some embodiments of the invention.

Referring to FIGS. 8A-8B an embodiment is illustrated in which separatetop electrode layers 118 are formed connecting one or more micro LEDdevices 400 with one or more ground tie lines 144. In the particularembodiment illustrated in FIGS. 8A-8B, the top electrode layers 118 onlyneed to provide the electrical path from a micro LED device 400 to anearby ground tie line 144. Accordingly, the top electrode layers 118 donot cover the entire pixel area 104. Likewise, top electrode layers 118are not required to cover the entire bank openings 128 or subpixelareas. In the particular embodiment illustrated, each top electrodelayer 118 connects a micro LED device 400 on opposite sides of anintermediate ground tie line 144. However, this particular configurationis exemplary and a number of different arrangements are possible. Asillustrated, the top electrode layer 118 may be formed within openings149 to the ground tie lines 144. In such an embodiment, since the groundtie lines 144 are in electrical connection with the ground ring 116, itis not necessary to form the top electrode layer 118 outside of thepixel area 104.

As illustrated, the ground ring 116 may be buried beneath anelectrically insulating layer such as such as insulator layer 146 inaccordance with the embodiments illustrated in FIGS. 8A-8B. In theparticular embodiments illustrated in FIG. 8A, topmost row of micro LEDdevices 400 are illustrated as being connected to the ground ring 116with individual top electrode layers 118. In such an embodiment, eachtop electrode layer 118 may contact the ground ring 116 through one ormore openings 130 as previously described. Accordingly, while theembodiments illustrated in FIGS. 8A-8B provide one manner for connectingthe micro LED devices 400 to ground tie lines 144 within the pixel area104, this does not preclude using separate top electrode layers 118 toconnect to the ground ring 116 without going through a ground tie line144.

In one aspect, the particular embodiment illustrated in FIGS. 8A-8B maybe particularly suitable for localized formation of the top electrodelayers 118 with ink jet printing or screen printing. Conventional AMOLEDbackplane processing sequences such as those used for the fabrication ofthe display panels in FIGS. 1-2 typically blanket deposit the topelectrode layer in deposition a chamber, followed by singulation of theindividual backplanes 100 from a larger substrate. In accordance withsome embodiments, the display panel 100 backplane is singulated from alarger substrate prior to transferring the array of micro LED devices400. In an embodiment, ink jet printing or screen printing provides apractical approach for patterning the individual top electrode layers118 without requiring a separate mask layer for each separate displaypanel 100.

While not illustrated separately it is to be appreciated that theembodiments illustrated in FIGS. 6A-6B, FIGS. 7A-7B and FIGS. 8A-8B arecombinable with the alternative opening configurations included in FIGS.3C-3D and passivation layer 148 configuration illustrated in FIG. 6C.

The formation of separate top electrode layer 118 as described abovewith regard to FIGS. 8A-8B may provide an additional benefit duringelectrical testing of the panel 100 after formation of the top electrodelayers 118. For example, it may not have been discovered prior toformation of the top electrode layers 118 that an underlying transistorhas malfunctioned and may always be ‘on’. One implication could be thatthe associated subpixel is always ‘on’ and emitting light. In theembodiment illustrated in FIG. 8C the top electrode layer 118 connectedto the defective TFT transistor is cut using a suitable technique suchas laser scribing to turn ‘off’ the subpixel. In this manner, curing theelectrical defect can be performed on the top surface of the TFTsubstrate rather than having to drill down to access the TFT transistor.

Referring now to FIGS. 9A-9D, embodiments of the invention may also beutilized to incorporate ground tie lines 134 into AMOLED display panels.Referring to the embodiments illustrated in FIGS. 9A-9B, a bottomelectrode layer 124 is formed on the planarization layer 122 inelectrical connection with the underlying TFT circuitry. In theembodiments illustrated ground tie lines 134 may be formedsimultaneously with the bottom electrode layer 124. Likewise, groundring 116 may be formed simultaneously with the bottom electrode layer124. Following the formation of the bottom electrode layer 124 andoptionally the ground tie lines 134 and/or ground ring 116, a pixeldefining layer 125 is formed including an array of subpixel openings 127and an array of ground tie line openings 132. An organic layer 120 isthen deposited over the subpixel openings 127. In an embodiment theorganic layer 120 is not deposited within ground tie line openings 132or on the ground ring 116. A top electrode layer 118 may then be formedover the patterned pixel defining layer within subpixel openings 127,within the openings 132 to make electrical contact with the ground tielines 134, and optionally over the ground ring 116. However, it is notrequired to make electrical contact with ground ring 116. In theembodiment illustrated in FIGS. 9C-9D, the top electrode layer 118 isonly deposited within the pixel area 104, and the top electrode layer1118 does not overlap the ground ring 116.

While not illustrated separately it is to be appreciated that theembodiments illustrated in FIGS. 9A-9D contemplate alternative openingconfigurations. For example, openings to the ground tie line 134 orground ring 116 could be made through the planarization layer 122.

FIG. 10 illustrates a display system 1000 in accordance with anembodiment. The display system houses a processor 1010, data receiver1020, a display panel 100, such as any of the display panels describedabove. The data receiver 1020 may be configured to receive datawirelessly or wired. Wireless may be implemented in any of a number ofwireless standards or protocols including, but not limited to, Wi-Fi(IEEE 802.11 family), WiMAX (IEEE 802.16 family), IEEE 802.20, long termevolution (LTE), Ev-DO, HSPA+, HSDPA+, HSUPA+, EDGE, GSM, GPRS, CDMA,TDMA, DECT, Bluetooth, derivatives thereof, as well as any otherwireless protocols that are designated as 3G, 4G, 5G, and beyond.

Depending on its applications, the display system 1000 may include othercomponents. These other components include, but are not limited to,memory, a touch-screen controller, and a battery. In variousimplementations, the display system 1000 may be a television, tablet,phone, laptop, computer monitor, kiosk, digital camera, handheld gameconsole, media display, ebook display, or large area signage display.

In utilizing the various aspects of this invention, it would becomeapparent to one skilled in the art that combinations or variations ofthe above embodiments are possible for integrating ground tie lines andmicro LED devices into an active matrix display panel. While the aboveembodiments have been described with regard to a top emission structure,embodiments of the invention are also applicable to bottom emissionstructures. For example, rather than locating the bank openings 128 orsubpixel openings 127 above the TFT circuitry, the openings could belocated adjacent the TFT circuitry on lower layers in the TFT substrate102. Similarly, while top gate transistor structures have beendescribed, embodiments of the invention may also be practiced withbottom gate transistor structures. Furthermore, while embodiments of theinvention have been described and illustrated with regard to a high sidedrive configuration, embodiments may also be practiced with a low sidedrive configuration in which the ground tie lines and ground ringdescribed above become the power line in the panel. Although the presentinvention has been described in language specific to structural featuresand/or methodological acts, it is to be understood that the inventiondefined in the appended claims is not necessarily limited to thespecific features or acts described. The specific features and actsdisclosed are instead to be understood as particularly gracefulimplementations of the claimed invention useful for illustrating thepresent invention.

What is claimed is:
 1. A display panel comprising: a ground line; adisplay substrate including a pixel area and a non-pixel area; an arrayof micro LED devices on the display substrate within the pixel area; anarray of bottom contacts formed on a planarization layer and underneaththe array of micro LED devices; an array of ground tie lines locatedbelow the planarization layer and running between the micro LED devicesin the pixel area and electrically connected to the ground line; and oneor more top electrode layers over and in electrical contact with thearray of micro LED devices and the array of ground tie lines.
 2. Thedisplay panel of claim 1, wherein the micro LED devices are verticalmicro LED devices.
 3. The display panel of claim 2, wherein eachvertical micro LED device comprises an inorganic semiconductor-based p-ndiode.
 4. The display panel of claim 3, wherein the micro LED deviceseach have a maximum width of 1 μm-100 μm.
 5. The display panel of claim3, further comprising a transparent passivation layer spanning sidewallsof the array of vertical micro LED devices, wherein the transparentpassivation layer does not completely cover a top conductive contact ofeach vertical micro LED device.
 6. The display substrate of claim 1,further comprising a bank layer on the planarization layer, the banklayer comprising an array of bank openings, wherein the array of microLED devices are within the array of bank openings.
 7. The displaysubstrate of claim 1, further comprising an array of via openings in theplanarization layer exposing the array of ground tie lines.
 8. Thedisplay substrate of claim 7, further comprising an array of conductivevia opening layers within the array of via openings and on the array ofground tie lines.
 9. The display panel of claim 8, wherein the one ormore top electrode layers comprises a single top electrode layer overand in electrical contact with the array of micro LED devices and thearray of conductive via opening layers.
 10. The display panel of claim8, wherein the one or more top electrode layers comprises an array ofelectrode layers over and in electrical contact with the array of microLED devices and the array of conductive via opening layers.
 11. Thedisplay panel of claim 3, wherein the one or more top electrode layerscomprises a single top electrode layer over and in electrical contactwith the array of micro LED devices and the array of ground tie lines.12. The display panel of claim 11, wherein the single top electrodelayer is formed of a transparent or semi-transparent material.
 13. Thedisplay panel of claim 11, wherein the single top electrode layercomprises a material selected from the group consisting of PEDOT andITO.
 14. The display panel of claim 3, wherein the one or more topelectrode layers comprises an array of electrode layers over and inelectrical contact with the array of micro LED devices and the array ofground tie lines.
 15. The display panel of claim 14, wherein each topelectrode layer electrically connects a plurality of micro LED devicesto a single ground tie line.
 16. The display panel of claim 14, whereinthe array of top electrode layers is formed of a transparent orsemi-transparent material.
 17. The display panel of claim 14, whereinthe array of top electrode layers comprises a material selected from thegroup consisting of PEDOT and ITO.